Recently, a demand for batteries as a power source has increased in response to development of portable devices such as personal computers and mobile phones. Batteries for these applications are expected to be used at normal temperatures, and to have a large energy density and good cycle characteristics.
In order to satisfy this demand, new active materials with large capacity density have been developed for a positive electrode and negative electrode, respectively. In particular, there is a great interest especially in using elemental silicon (Si) or tin (Sn), or oxides thereof providing extremely large capacity density as a negative active material.
However, rechargeable lithium batteries where these materials are used as the negative active materials do not exhibit sufficient charge/discharge cycle characteristics. Specifically, silicon oxide (SiO0.3) is used as a negative active material. A positive electrode of lithium cobalt oxide (LiCoO2), which is generally used for rechargeable lithium batteries, is used as a positive active material. As electrolyte, a mixed solution of ethylene carbonate (EC) and ethylmethylcarbonate (EMC) containing lithium hexafluorophosphate (LiPF6) is used. These materials are used for manufacturing a winding type rechargeable lithium battery. When this battery is repeatedly charged and discharged under the condition: charge/discharge current of 1.0 C; charge cutoff voltage of 4.2 V; and discharge cutoff voltage of 2.5 V, its capacity significantly reduces at about 100-th cycle.
In order to suppress this capacity reduction, it is preferable to minimize the discharge potential of the negative electrode. For example, Patent Document 1 discloses that the discharge cutoff potential of the negative electrode is controlled to 0.6 V or lower with respect to a lithium reference electrode in a rechargeable lithium battery using silicon oxide (SiO) as the negative active material. Such control suppresses the capacity reduction in the charge/discharge cycles.
When the lithium reference electrode is provided in the battery in order to control the discharge potential of the negative electrode made of SiO with respect to the lithium reference electrode as in Patent Document 1, however, a battery structure becomes complicated. Therefore, practical use of the rechargeable lithium battery of Patent Document 1 is difficult. In other words, the battery requires a third electrode terminal, in addition to positive and negative electrode terminals.
In the initial period of the charge/discharge cycles, deterioration in positive and negative active materials is small. When the charge/discharge cycle is repeated from this state, the respective potential ranges in which positive and negative electrodes are used vary with deterioration of positive and negative active materials. Therefore, even when the discharge cutoff voltage is set so that the discharge cutoff potential of the negative electrode is 0.6 V or lower with respect to the positive electrode, it is difficult to determine whether the discharge cutoff potential of the negative electrode is actually maintained at 0.6 V or lower. Accordingly, it is difficult to detect the charge/discharge state of the negative electrode by measurement of the battery voltage and suppress the capacity reduction in the charge/discharge cycles.    Patent Document 1: Japanese Patent Unexamined Publication No. H11-233155